1. Field of the Invention
The present invention relates generally to the chemistry of cyanide compounds and more specifically to a method for decomposing the cyanide radical in those compounds. The method is particularly well-suited for treating high concentration, cyanide-bearing hazardous wastes, such as those generated by electroplating, stripping and heat treating operations.
2. Description of the Prior Art
As used herein, the term "cyanide compound" means any chemical compound which includes, as a part of its structure, one or more cyanide radicals. The cyanide radical is characterized by a triple-bonded arrangement of carbon and nitrogen (in a one-to-one atomic ratio), which generally behaves in a monovalent fashion. The cyanide radical may bond in a covalent manner and form a part of another radical, such as in the case of a cyanate, or may bond in an ionic manner, such as in the case of a cyanide salt. Thus, cyanide compounds include nitriles, cyanates, cyanogen, cyanamide, hydrogen cyanide, complex metal cyanides (such as ferrocyanides), alkali-metal cyanides, ammonium cyanide and alkaline-earth metal cyanides.
As used herein, the term "cyanide-bearing" waste, or material, means a waste product or a material which includes a substantial amount of concentration of one or more cyanide compounds. A substantially pure, or even reagent grade cyanide compound is, therefore, included in this definition. Because cyanide compounds are so highly toxic, a concentration of only a few parts per million (ppm), by weight of the cyanide radical, may be considered substantial. Thus, relatively low concentration wastes and materials are also considered herein to be "cyanide-bearing".
It follows then, by definition, that a cyanide-bearing waste or material contains a substantial quantity or concentration of cyanide radical. The presence of the cyanide radical is the only distinguishing feature of a cyanide-bearing material or waste, regardless of any other compounds or elements which may be chemically associated or bonded with the cyanide radical. Likewise, for purposes of this invention, the composition of the bulk waste or material which carries the cyanide compound is largely irrelevant, so long as it does not prevent the decomposition of the cyanide radical in accordance with the present invention. Unless otherwise specifically designated, the term "cyanide concentration", as used herein, will always refer to the weight of cyanide radical, in the cyanide-bearing waste or material, expressed in parts per million.
A large number of chemical processes, especially in the metals industries, generate cyanide-bearing wastes. Heat treating, precious metals recovery from ore, electroplating and stripping of plated deposits are some of the activities which often produce such hazardous wastes.
In the heat treatment and case hardening of ferrous alloys, molten salt baths, which may contain sodium, calcium and potassium cyanide, are often employed. The salt baths cause the formation of a high hardness case, which includes carbides and nitrides. The molten salt bath also brings the workpiece above the austenitizing temperature of most steels, thereby permitting additional hardening by quenching in an oil or water bath. This results in substantial amounts of cyanide salts being carried over to the quenching solution. Cyanide concentrations of ten to thirty thousand ppm are typical.
Many electroplating processes employ cyanide salts in the electrolyte. After plating, the workpiece is washed by immersion in a plurality of rinse water tanks. Residual cyanide electrolyte (termed "drag-out") is carried out by the workpiece and diluted in the rinse water.
The drag-out rinse tanks are frequently fed with a counter-current flow of fresh water. For example, in a three tank arrangement, the drag-out concentration is highest in the first tank, lower in the second and lowest in the third or final tank. Fresh rinse water is introduced only to the final rinse tank. As the final tank overflows, the effluent is directed to the second tank, which overflows to the first tank. The relatively high concentration overflow from the first tank is diverted for cyanide destruction and the removal of dissolved metals.
Typically, the cynanide concentration is electroplating rinse waters is about two to three hundred ppm. Prior to disposal, most municipal sewage treatment facilities require that concentration be brought down to an upper limit, usually set between one and ten ppm. Often, alkaline chlorination is used to decompose the cyanide radical. This process is carried out by first adjusting the pH of the solution and then bubbling with chlorine gas or making additions of sodium hypochlorite. A polymer flocculent may be added to the waste water stream as it is diverted to a large settling tank termed a "clarifier". The flocculent causes agglomeration of the insoluble metal compounds which then settle as a sludge. The sludge, which contains substantial quantities of residual cyanide (one thousand ppm is not uncommon) is then dewatered in a filter press and sent out for land fill disposal.
Highly concentrated solutions of cyanide (typically between about 30,000 and 100,000 ppm) are used to strip plated deposits of various metals from workpieces. Stripping may be used when a substandard or defective plate is applied to an otherwise salvageable workpiece. Examples include tubular frame furniture and automobile bumpers, where a smooth nickel underlay is necessary prior to chromium plating.
The need for stripping is also encountered when a ferrous workpiece requires electroplating subsequent to gas carburizing. The fastener industry fabricates a type of bolt which has a washer permanently affixed to the bolt head or slidably affixed to the bolt shank. It is often desirable to case harden the bolt but not the washer. The washer can be isolated from the carburizing gas by selectively electroplating it with a thin deposit of copper. Because carbon has a very low solubility in copper, even at carburizing temperatures, the washer does not case harden.
After carburizing, the bolt and washer units are sent to the electroplater for finishing. It is generally necessary for the electroplater to strip the copper deposit from the washer before applying the finish plate.
Many types of stripping solutions are commercially available. Some, which work quite rapidly, employ a reverse or "deplating" direct current to facilitate stripping. Others, which generally strip at a slower rate, rely only on the chemical action of the solution to dissolve the plate. Both processes have the drawback that they often cause some of the base metal from the workpiece to go into solution. In the case of ferrous alloy base metals, this may cause the formation of complex ferrocyanides, which are not readily decomposed by prior art methods of cyanide destruction. It is believed that the use of a deplating current exacerbates this problem.
Once the stripping solutions are saturated with metals, or become "spent", they must be disposed of. Some electroplaters trickle the spent stripping solution into the drag-out rinse overflow and treat by chlorination. The disadvantage of this method is that the stripping solution must be added slowly, so that the cyanide concentration does not exceed the ability of the chlorination process to destroy it. In addition, this method substantially increases the amount of chlorination (and therefore the cost) required to treat the waste water.
Many independent waste treatment facilities have been constructed to handle cyanide-bearing wastes. While the processes employed by those treaters are unknown to the present inventors, it has been reported to them by some electroplaters that prices of up to fourteen dollars per gallon are currently being charged for the treatment of spent stripping solutions. It has also been reported that many waste treaters refuse to accept high concentration cyanide-bearing wastes, at any price.
Other methods have been reported as capable of removing cyanide from aqueous waste solutions. Some of these methods precipitate the cyanide by forming a relatively insoluble compound, such as is disclosed in U.S. Pat. No. 4,543,189 Rice et al. REMOVAL OF COMPLEXED ZINC-CYANIDE FROM WASTEWATER. The precipitate, however, presents a new disposal problem as it comprises concentrated cyanide compounds.
Other methods have been developed to decompose cyanide ions in solution by employing electrolysis. (See for example U.S. Pat. No. 4,417,963 Janne, PROCESS FOR REMOVING CYANIDE IONS FROM SOLUTIONS.)
Still another method of decomposing cyanides has been proposed by the Ontario Research Foundation, Sheridan Park Research Community, Mississauga, Ontario, Canada L5K 1B3. This method is disclosed in a report prepared for The American Electroplater's Society, 1202 Louisiana Ave., Winter Park, Fla. 32789, designated as AES Research Project 53, Final Report P-3083/I-3. The report was authored by A. P. Cadotte and bears a date of June 10, 1980.
The Ontario Research Foundation suggests that cyanide can be broken down to ammonia and formic acid by hydrolysis conducted at elevated temperatures and pressures. Specifically, the report recommends treating aqueous cyanide solutions in the liquid phase and at a temperature between about 190.degree. C. and 275.degree. C. Corresponding pressures range between about 250 and 900 psig.
On the basis of laboratory trials, the removal of up to 99.998% of the cyanide from high concentration solutions (defined therein as about 50 grams of cyanide per liter) and 99.9% from low concentration solutions (defined therein as 100 milligrams cyanide per liter) was predicted. The suggested residence times in the reaction vessel were, however, as much as 6 to 8 hours for high concentration solutions and 2 to 3 hours for low concentration solutions.
The drawbacks of this process are numerous. A large capital investment is necessary to construct a reaction vessel capable of operating at the suggested temperatures and pressures. The rate of cyanide decomposition is relatively slow and extreme safety precautions must be taken to ensure that such a reaction vessel retains its structural integrity during repeated use.